JP2661002B2 - Intermediate frequency filter circuit and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency tracking type intermediate frequency filter circuit in an FM receiver and a method of manufacturing the same.

[0002]

2. Description of the Related Art A frequency tracking type intermediate frequency filter circuit in an FM receiver is generally configured as a bandpass filter by connecting a coil 1, a fixed capacitor 2, and a variable capacitance element 3 in parallel as shown in FIG. And F
The output voltage of the AFC circuit obtained from the M demodulation circuit is
In addition to the variable capacitance element 3, the tuning frequency of the filter is made to follow the frequency of the input signal. Reference numeral 4 denotes a capacitor for cutting direct current.

Generally, in an intermediate frequency filter circuit which is not a frequency tracking type, a band is set relatively wide so as to cover all FM modulated waves whose frequency always changes.
When an input signal is applied to the intermediate frequency filter circuit, it is often the case that the adjacent signal interferes with the desired signal because the signal adjacent to the desired signal cannot be sufficiently removed.

A frequency tracking type intermediate frequency filter circuit is
The bandwidth can be made very narrow by continuously tuning the tuning frequency to the constantly changing frequency in the intermediate frequency band of the desired signal, so that it is effective to select the desired signal from among the interfering signals. Has the advantage of being
In recent years, it has been rapidly adopted. In this case, in order to obtain a correct FM demodulation output, it is necessary to make the tuning frequency of the filter circuit accurately follow the frequency of the input signal that changes within the band of the intermediate frequency, and add it to the demodulation circuit again.

The tuning frequency f of the intermediate frequency filter circuit shown in FIG. 5 is expressed by the following equation (1). f = 1 / 2π ｛L (C + C _V )｝ ^1/2 (1) where L is the inductance value of the coil 1, C is the capacitance value of the capacitor 2, and C _V is variable when the control voltage of the terminal 5 is V. This is the capacitance value of the capacitance element 3.

[0006] The frequency of the input signal of the intermediate frequency filter circuit changes in the frequency range of f _{1 to} f ₂ , and the frequencies f _{1 to} f
When the control voltage from the AFC circuit corresponding to ₂ varies from V ₁ ~V _2, to the filter circuit to accurately follow the input signal satisfies the following formula (2) and (3) There is a need. f ₁ = １ ／ π ｛L (C + C _V1 )｝ ^1/2 (2) f ₂ = １ ／ π ｛L (C + C _V2 )｝ ^1/2 (3) where C _V1 and C _V2 are control voltages V ₁ , V _{2 at} the time of the variable capacitance element 3.

As is apparent from the equations (2) and (3),
When the capacitance value C and the inductance value L are fixed,
The capacitance value C _V1 and the capacitance value C _V2 satisfying the expressions (2) and (3)
Has only one set. When the inductance value L can be varied, it can be adjusted so as to satisfy either of the equations (2) and (3). To satisfy the remaining equations,
It is necessary to satisfy the expression (4) obtained from the expressions (2) and (3). (F ₁ / f ₂ ) ² = (C + C _V2 ) / (C + C _V1 ) (4)

However, the variation of the capacitance value C _V1 and the capacitance change ratio C _V1 / C _V2 of the variable capacitance element 3 which normally uses a varicap diode is ± 20% and ± 10%, respectively.
When converted to the variation of the frequency change ratio f ₁ / f ₂ , it becomes ± 5% or more. On the other hand, the bandwidth of the intermediate frequency filter circuit is set to ± 2% or less of the tuning frequency when the band is narrow.

Therefore, the input signal cannot be accurately followed, and the input signal cannot be partially passed, and a defective product which cannot perform the function of the filter circuit is likely to be formed. If only the variable capacitance element 3 having the capacitance value C _V1 and the capacitance value C _V2 satisfying the expression (4) is selected and used, the cost will be significantly increased, and supply will be uneasy. .

Of course, the capacitance values of the capacitors C are classified in advance, and the capacitance values C _V1 and C
Combinations can be changed according to the variation of _V2 , but it is very difficult to constantly deal with all possible combinations because inventory management and manufacturing management of the capacitor 2 and the variable capacitance element 3 become complicated. It is. In the conventional frequency tracking type intermediate frequency filter circuit, it is difficult to obtain a stable and economical filter circuit because of the variation in the capacitance value of the variable capacitance element used in this manner.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency tracking type intermediate frequency filter circuit which can be stably obtained at an economical price and a method of manufacturing the same.

[0012]

SUMMARY OF THE INVENTION The present invention relates to a frequency tracking type intermediate frequency filter for an FM receiver which is equivalently connected in parallel with an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor. In the circuit, a tap is provided in the coil, one of a fixed capacitor and a variable capacitance element is connected between the tap and one end of the coil, and the other is connected between both ends of the coil, according to the frequency of the input signal. Thus, the ratio of the combined capacitance value in the filter circuit at any two points within the range of the control voltage applied to the variable capacitance element becomes substantially equal to the reciprocal of the square of the ratio of the frequency of the input signal at the two points. Thus, the electrical turns ratio is set between one end and tap of the coil and between both ends. The coils were joined 2 instead of tapping.
One coil may be used. The present invention further resides in a method for manufacturing such an intermediate frequency filter circuit.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an intermediate frequency filter circuit according to the present invention will be described below with reference to FIGS. 1 is a circuit diagram, and FIG. 2 is an equivalent circuit diagram. The same parts as those in FIG. 5 are denoted by the same reference numerals. In FIG. 1, the coil 1 having one end grounded is provided with a tap 6, and the fixed capacitor 2 is connected between the tap 6 and the ground.

A series circuit of a variable capacitance element 3 and a capacitor 4 is connected between both ends of the coil 1. A terminal 5 for applying a control voltage of the variable capacitance element 3 is connected to a connection point between the variable capacitance element 3 and the capacitor 4. In addition, the number of turns of the coil 1 is represented as 1 and the number of turns between the tap 6 and one end grounded is represented as N. The number of turns N represents the ratio of the number of turns between the tap 6 and one end grounded at the same time and the number of turns between both ends of the coil 1.

FIG. 2 is an equivalent circuit diagram of the circuit of FIG.
Inductance L of the coil 1 in Figure 1, the capacitance value C ₁ of the fixed capacitor 2, the capacitance value C _V of the variable capacitance element 3 when the control voltage V applied to the terminal 5 are connected in parallel equivalently. The capacity value C ₁ of the fixed capacitor 2 and the capacitance value at the time of FIG. 1 is C, a relation of equation (5). C ₁ = N ² · C (5)

Accordingly, the combined capacitance value of the filter circuit viewed from both ends of the coil 1 is as shown in the following equation (6). C ₁ + C _V = N ² · C + C _V (6) Note that the capacitor 4 serves to cut off the control voltage V and can be ignored since the capacitance value C _C is increased.

The frequency of the input signal to the filter circuit is f ₁
The control voltage changes from V ₁ to V ₂ in response to the change from V ₁ to f _2, and the capacitance value of the variable capacitance element 3 changes from C _V1 to C 2.
_When it changes to _V2, equation (7) is established in the same manner as equation (4) of the conventional circuit in FIG. (F ₂ / f ₁ ) ² = (N ² · C + C _V1 ) / (N ² · C + C _V2 ) (7) Equation (8) is obtained from equation (7). N = [(α · C _V2 −C _V1 ) / {C (1−α)}] ^1/2 (8) where α = (f ₂ / f ₁ ) ² .

Accordingly, the number of turns N between the tap 6 of the coil L and one end grounded can be set by the equation (8).
That is, even if the capacitance value of the variable capacitance element 3 varies,
Adjusting the position of the tap prevents the tuning frequency of the filter circuit from deviating from the frequency of the input signal to be followed. That is, it is possible to absorb the adverse effect of the variation in the capacitance value of the variable capacitance element 3.

The positions of the taps are set so that the ratio of the combined capacitance of the filter circuit at the two points of the frequency of the input signal is equal to the reciprocal of the square of the ratio of the frequencies at the two points, as can be seen from equation (8). Is done. That is, the variable ratio of the combined capacitance of the filter circuit is made constant. These two points are (8)
Although the formula is set to the frequency f _1, f ₂ of the end of the input signal, set on the inside or outside of the frequency of the input signal instead of the frequency f _1, f ₂ is an extension of the frequency f _1, f ₂ You can also. As a result, the position of the tap can be obtained from equation (8).

[0020] Then, the filter circuit is set variable range of the input signal, that the inductance value L of the coil 1 to tune a frequency of a point between the frequency f ₁ of the frequency f ₂ from (9) . f ₁ = 1 / 2π ｛L (N ² · C + C _V1 )｝ ^1/2 (9) The frequency at one point is set to the frequency f ₁ at the end of the input signal in the equation (9). since the variable ratio of the combined capacitance of the filter circuit as described above is set to be constant, the frequency f ₂ from the frequency f ₁ of the input signal by tuning in one of frequencies between the frequency f ₁ of the frequency f ₂ Can be made to follow the filter circuit within the range. However, the capacitance value of the variable capacitance element 3 changes according to the set frequency of one point. For example, when the frequency f ₂ in place of the frequency f ₁ is set to C _V2.

As described above, the variation in the capacitance value of the variable capacitance element 1 is absorbed by adjusting the position of the tap 6, and the filter circuit is tuned at one point within the range in which the frequency of the input signal changes. , The tuning frequency of the filter circuit can accurately follow the input signal.

In manufacturing the intermediate frequency filter circuit of the present invention, the position of the tap 6 may be adjusted for each filter circuit according to equation (8). The number of turns N can be determined, and the number of turns of another filter circuit can be determined based on the number of turns N. Control voltage according to the change from f ₁ frequency to f ₂ of the input signal of the first filter circuit V as a reference
₁ to V _2, and the capacitance value of the variable capacitance element 3 becomes C _V1
When changes to C _V2 from the first filter circuit (7) is established.

In the same frequency range of the input signal, another second
When the capacitance value of the variable capacitance element 3 of the filter circuit changes from C _V11 to C _V22 , Expression (10) is established. (F ₂ / f ₁ ) ² = (M ² · C + C _V11 ) / (M ² · C + C _V22 ) (10) where M is the number of turns between the tap 6 of the coil 1 in the second filter circuit and one end grounded. And the number of turns between the tap 2 and one end grounded at the same time, and between both ends of the coil 1.

As is apparent from equations (7) and (10), the ratio of the combined capacitance value of the first filter circuit at the frequency f ₁ and the frequency f ₂ is the combined capacitance value of the second filter circuit at the same frequency. , The following equation (11) is obtained. Equation (11) becomes like equation (12).

[0025]

(Equation 1)

Where β = C _V1 / C _V2 , β ₁ = C _V11 /
C _V22 . In this manner, the position of the tap 6 of another filter circuit can be set with reference to the position of the tap 6 of the first filter circuit as a reference in accordance with the variation in the change ratio of the capacitance of the variable capacitance element. . This method is a very effective and practical means for obtaining stable characteristics when a large number of filter circuits are manufactured. In other words, in the coil manufacturing process centering on the customer specification, the number of turns can be set freely, and the variable capacitance elements to be combined are classified into ranks according to the change ratio of the capacitance, and the taps are set in units of each rank. Is convenient because it can be set optimally.

The method for manufacturing an intermediate frequency filter according to the present invention comprises:
By setting the position of the tap 6 by one of the equations (11) and (12), it is possible to absorb the adverse effect of the variation in the capacitance value of the variable capacitance element 3 on the tuning frequency of the filter circuit.

Then, the inductance value L of the coil 1 is set by the equation (13). Frequency of one point, have selected the frequency f ₁ in the same way as equation (9). f ₁ = 1 / 2π ｛L (M ² · C + C _V11 )｝ ^1/2 (13) As described above, the position of the tap 6 of one filter circuit is adjusted with reference to the position of the tap 6 of another filter circuit. May be.

Although the variation in the change ratio of the capacitance of the variable capacitance element can be adjusted by the position of the tap 6, the Q of the coil 1 also changes in order to set the inductance according to the capacitance value. This leads to a change in the bandwidth of the filter circuit. Therefore, contrary to the order described, (13)
If the inductance value L is obtained in advance by the equation or the equation (9) and the total number of turns of the coil 1 is set so that the optimum Q value is obtained by the inductance value L, more stable characteristics can be obtained. Can be Also in this case, the position of the tap 6 remains set by the equations (8), (11), and (12).

FIG. 3 is a circuit diagram showing another embodiment of the intermediate frequency filter circuit of the present invention, and the same parts as those in FIG. 1 are denoted by the same reference numerals. In FIG. 3, the fixed capacitor 2 is connected between both ends of the coil 1 whose one end is grounded, and the variable capacitance element 3 is connected between the tap 6 of the coil 1 and one end. A terminal 5 to which a control voltage for controlling the capacitance value of the variable capacitance element 3 is applied is connected to a connection point between the variable capacitance element 3 and a capacitor 4 for cutting DC.

Even with such a configuration, the inductance value L of the coil 1, the capacitance value C of the capacitor 2, the variable capacitance element 3
The capacitance value C _V is connected in parallel equivalently. In such a configuration, assuming that the total number of turns of the coil 1 is N and the number of turns between the tap 6 and one end thereof is 1, the position of the tap 6 and the inductance value L may be obtained as in the first embodiment.
Alternatively, the tap 6 of the coil 1 of the filter circuit serving as a reference may be set, and the number of turns M of the tap 6 of another filter circuit may be set based on the number of turns N of the coil 1 at the tap 6.

FIG. 4 is a circuit diagram showing still another embodiment of the intermediate frequency filter circuit of the present invention. In FIG. 4, the variable capacitance element 22 is connected between both ends of the coil 21 and the coil 2
1 and a fixed capacitor 24 between both ends of the coil 23
Is connected. In the embodiment of FIG.
The inductance value composed of 1 and the coil 23, the capacitance value of the capacitor 24, and the capacitance value of the variable capacitance element 22 are equivalently connected in parallel.

In such a configuration, the number of turns of the coil 21 may be set to 1, the number of turns of the coil 23 may be set to N, and the number of turns N of the coil 23 may be set instead of the tap. The number of turns in the embodiment is a number of turns for realizing a turn ratio equal to the voltage conversion ratio, and means an electric turn number.

[0034]

As described above, in the intermediate frequency filter circuit of the present invention, the inductance value of the coil, the capacitance value of the fixed capacitor, and the capacitance value of the variable capacitance element are equivalently connected in parallel. By setting the electrical turns ratio between one end and both ends, the configuration is such that the variation in the capacitance value of the variable capacitance element can be absorbed. A coupling coil may be used instead of providing a tap.

By setting the inductance value of the coil so as to tune at one point of the input signal, it is possible to accurately follow the frequency of the input signal regardless of the variation in the capacitance value of the variable capacitance element. Unlike the conventional case, it is not necessary to select a variable capacitance element having a fixed characteristic, which is extremely economical. Further, since the electrical turns ratio of the coil of another second filter circuit can be set based on the first filter circuit serving as a reference, there is an advantage that the manufacturing method is simplified.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of an intermediate frequency filter circuit of the present invention.

FIG. 2 is an equivalent circuit diagram of FIG.

FIG. 3 is a circuit diagram showing another embodiment of the intermediate frequency filter circuit of the present invention.

FIG. 4 is a circuit diagram showing still another embodiment of the present invention.

FIG. 5 is a circuit diagram of a conventional intermediate frequency filter circuit.

[Explanation of symbols]

 1 coil 2 fixed capacitor 3 variable capacitance element 6 tap

Claims (8)

(57) [Claims] 1. An intermediate frequency filter circuit of a frequency tracking type of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. One of a fixed capacitor or a variable capacitance element is connected between the tap and one end of the coil, and the other is connected between both ends of the coil. An intermediate frequency filter circuit characterized in that the ratio of the number of turns is adjusted so that the variable ratio of the combined capacitance of the filter circuit within the range of the control voltage applied to the variable capacitance element is constant. 2. A frequency tracking type intermediate frequency filter circuit of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. One of a fixed capacitor and a variable capacitance element is connected between the tap and one end of the coil, and the other is connected between both ends of the coil, and a control is applied to the variable capacitance element according to the frequency of the input signal.
Filter for voltage values near the upper and lower ends of the control voltage
The ratio of the combined capacitance value of the circuit is near the upper end and the lower end.
About the reciprocal of the square of the frequency ratio of the input signal
As described above, the electrical connection between one end of the coil and the tap and between the two ends
An intermediate frequency filter circuit, wherein a target turns ratio is set . 3. A frequency tracking type intermediate frequency filter circuit of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. A first coil to which the variable capacitance element is connected, and a second coil to which a fixed capacitor is connected between both ends and which is coupled to the first coil; and the number of electrical turns of the first coil and the second coil An intermediate frequency filter circuit wherein the ratio is adjusted so that the variable ratio of the combined capacitance of the filter circuit within a range of a control voltage applied to the variable capacitance element is constant. 4. An intermediate frequency filter circuit of a frequency tracking type of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. a first coil variable capacitance element is connected, is fixed capacitor connected across, made from a second coil coupled with the first coil, response to the frequency of the input signal
Near and below the upper end of the control voltage applied to the variable capacitance element
Ratio of the combined capacitance value of the filter circuit at the voltage value near the edge
Is the frequency of the input signal near the upper end and the lower end.
The first coil so that it is approximately equal to the inverse of the square of the ratio
And an electric turns ratio of the second coil and the second coil . 5. An intermediate frequency filter circuit of a frequency tracking type of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitor, and a capacitance value of a fixed capacitor are equivalently connected in parallel. One of a fixed capacitor and a variable capacitance element is connected between the tap and one end of the coil, and the other is connected between both ends of the coil, and a control is applied to the variable capacitance element according to the frequency of the input signal.
Filter for voltage values near the upper and lower ends of the control voltage
The ratio of the combined capacitance value in the circuit is near the upper end and the lower end.
The reciprocal of the square of the ratio of the frequency of the input signal
So that the voltage between one end of the coil and the tap and between both ends is
An intermediate frequency filter, wherein a magnetic turns ratio is set , and an inductance value of the coil is set so that the filter circuit is tuned substantially at the center of the frequency of the input signal. 6. An intermediate frequency filter circuit of a frequency tracking type of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. a first coil variable capacitance element is connected, is fixed capacitor connected across, made from a second coil coupled with the first coil, response to the frequency of the input signal
Near and below the upper end of the control voltage applied to the variable capacitance element
Of the combined capacitance value in the filter circuit at the voltage value near the end
The ratio is the frequency of the input signal near the upper end and near the lower end.
The first coil is approximately equal to the reciprocal of the square of the ratio
An intermediate frequency filter , wherein an electric turns ratio of the first coil and the second coil is set, and an inductance value of the coil is set so that the filter circuit is tuned substantially at the center of the frequency of the input signal. . 7. A frequency tracking type intermediate frequency filter circuit of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. And one of a fixed capacitor and a variable capacitance element is connected between the tap and one end of the coil, and the other is connected between both ends of the coil, and is added to the variable capacitance element according to the frequency of the input signal.
At the voltage values near the upper and lower ends of the control voltage
The ratio of the combined capacitance value of the filter circuit is near the upper end and the lower end.
About the reciprocal of the square of the frequency ratio of the input signal
Between the tap and one end of the coil, and between the two ends.
In a method of manufacturing an intermediate frequency filter circuit in which an electric turns ratio is set and an inductance value of the coil is set so that the filter circuit is tuned substantially at the center of the frequency of the input signal, The electric turns ratio of the first filter circuit is set, and the capacitance value of the variable capacitance element at the voltage values near the upper end and the lower end selected by the first filter circuit is equal to the capacitance value of the variable capacitance element of the first filter circuit. When manufacturing different second filter circuits, the ratio of the combined capacitance value of the second filter circuits near the upper end and the lower end becomes equal to the ratio of the combined capacitance of the first filter circuits near the upper end and the lower end. A method of manufacturing an intermediate frequency filter circuit, wherein the electrical turns ratio is set as described above. 8. A frequency tracking type intermediate frequency filter circuit of an FM receiver in which an inductance value of a coil, a capacitance value of a variable capacitance element, and a capacitance value of a fixed capacitor are equivalently connected in parallel. A first coil connected to the variable capacitance element, and a fixed capacitor connected between both ends,
Made from a second coil coupled with the first coil, the input signal
Control voltage applied to the variable capacitance element according to the frequency of the signal
Of the filter circuit at the voltage values near the upper and lower ends of
When the ratio of the combined capacitance value is the input near the upper end and the input near the lower end,
Approximately equal to the reciprocal of the square of the frequency ratio of the signal
, Set the electrical turns ratio of the first coil and the second coil
And further the inductance of the coil, in the manufacturing method of the intermediate frequency filter circuit filter circuit is set to tune almost at the center frequency of the input signal, the electrical of the first filter circuit comprising a pre-reference A turn ratio is set, and the capacitance value of the variable capacitance element at the voltage values near the upper end and the lower end selected by the first filter circuit is different from the capacitance value of the variable capacitance element of the first filter circuit. In manufacturing, the electric turns ratio is set such that the ratio of the combined capacitance of the second filter circuit near the upper end and the lower end is equal to the ratio of the combined capacitance value of the first filter circuit near the upper end and the lower end. A method for manufacturing an intermediate frequency filter circuit, comprising: